1. Technical Field
The present invention relates to a method and a system for calculating the position of a moving vehicle.
2. Related Art
GPS (Global Positioning System) is widely known as a positioning system which uses a positioning signal and is used in a position calculation device that is provided in a portable phone, a car navigation device, and the like. In the GPS, a position calculation computation is performed so as to calculate the position coordinates and timepiece errors of the position calculation device based on information such as the positions of a plurality of GPS satellites and a pseudo-range from each GPS satellite to the position calculation device.
A GPS satellite signal transmitted from a GPS satellite is modulated with spread codes, called CA (Coarse and Acquisition) codes, different for each GPS satellite. The position calculation device performs a correlation operation between the received signal and replica CA codes which are the replica of the CA codes to capture the GPS satellite signal based on the correlation value. In this case, when the GPS satellite signal is captured from weak received signals, in order to easily detect the peak of the correlation value, a method of integrating the correlation values acquired through the correlation operation over a predetermined integration period is used.
However, since the CA codes themselves which spread-modulate the GPS satellite signal are also subjected to BPSK (Binary Phase Shift Keying) modulation every 20 ms with a navigation message, the polarities of the CA codes are reversed every 20 ms. Therefore, when the correlation values are integrated according to this reversal time, there is a problem in that correlation values of different signs are integrated. As a technique for solving this problem, JP-A-2001-349935 discloses a technique of integrating the correlation values based on polarity reversal timing information acquired from the outside.
According to the technique of JP-A-2001-349935, it is possible to increase the correlation integration period to be longer than 20 ms. However, increasing the correlation integration period causes other problems. One such problem is the Doppler effect. For example, if the moving velocity of the position calculation device changes, the Doppler frequency changes. As a result, the search frequency may gradually change during the long correlation integration period and eventually be located out of the initial frequency search range. For example, when the position calculation device is moving at an acceleration of 1 [G], a variation of the Doppler frequency is 50 [Hz/s]. That is, if the moving velocity changes, the actual search frequency will deviate from an initially intended search frequency.
In theory, if the Doppler frequency is not changed, the correlation integration value for a specific frequency becomes large as the integration of the correlation values proceeds. Thus, the specific frequency (=receiving frequency) can be easily identified by the peak of the correlation integration value. However, in practice, since the Doppler frequency changes, there is a shift between the actual search frequency and the initially intended frequency, and the specific frequency itself will change. As a result, paradoxically, it is difficult to identify the peak of the correlation integration value if the correlation integration is performed for a predetermined period or longer.
Thus, it may not possible to capture the satellite signal even when the correlation integration period is increased.